Problem: Which of these numbers is prime? ${37,\ 52,\ 55,\ 69,\ 81}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 37 are 1 and 37. The factors of 52 are 1, 2, 4, 13, 26, and 52. The factors of 55 are 1, 5, 11, and 55. The factors of 69 are 1, 3, 23, and 69. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 37 is a prime number.